Enable one Active popuup at a time

We are working on a Health Contact Center Application in which a
patient is able to contact using either Chat Application or Phone Call
with the Doctors.When the doctor is contacted using either of these
means, a popup appears at the Doctor's End and he is able to
communicate with the patient. When a doctor is busy handling the Phone
Call ( a windows form), he should not be able to work with the Chat
Request(less Priority) but when he is working with the Chat
Application,he can work on the phone Call(high Priority). So we want
to have some mechanism in which if these two popups(Phone and Chat
Window) are open at the sametime,doctor should be able to work on
Phone Call only and Focus Should not Go to Chat Window.